Files released this autumn at the National Archives in Kew include one dossier showing how the Special Operations Executive - Churchill's "secret army" - was not disbanded at the end of the Second World War, as is commonly thought.
Instead, nearly 300 agents were brought into the Secret Intelligence Service - otherwise known as MI6.
Officially SOE, the organisation set up to run resistance in occupied countries, was "liquidated" in January 1946.
SOE operatives were trained in all aspects of clandestine operations. They organised sabotage, guerrilla actions, black propaganda operations and financial warfare. They were active in Europe, the Middle East, and Asia.
SOE researchers had perfected secret communications and even developed weapons such as exploding rats.
Training schools, like the one at Beaulieu in Hampshire, taught agents all manner of spy techniques, including how to kill an opponent with their bare hands.
When peace came, some in Churchill's government argued that SOE could have a valuable role. Lord Selborne, the minister responsible, suggested it could be used to run clandestine operations in communist Europe and elsewhere. But the incoming Labour Government was less enthusiastic. It was Clement Attlee who oversaw its demise.
The recently released file reveals the liquidation was a front. One briefing paper for the Chiefs of Defence Staff from January 1946 shows that while the "liquidation party" at SOE's headquarters was closing down the office, 280 of its personnel were becoming "Special Operations Branch".
The plan was for them to be headed by "C" - the head of SIS - but to work independently.
SOE agents were trained in secret locations around Britain
However, in just a matter of weeks "C" - Sir Stewart Menzies - had decided that the idea of a separate branch was "unsound". He told the Chiefs of Defence Staff that he believed it would be better simply to merge them.
Though in wartime SIS and SOE had sometimes worked well together, they were in many ways quite different.
SIS had been founded in 1909, to secretly acquire intelligence. SOE - founded in 1940 - was far brasher, by the end of the war far bigger, and operated in a way that was closer to the James Bond idea of the secret agent.
So did merging the two mean the transformation of the secret service?
Dr Philip Davies, author of "MI6 and the Machinery of Spying", knew about the merger - though he had not seen the detailed manpower figures - and considers it more of a takeover.
He says that it was the acquisition of the training and research staff which was probably most significant, as SIS had few facilities in this area.
When it came to operations, Sir Stewart Menzies made sure that any decisions were left to SIS staff, and the Foreign Office had to approve foreign actions, so the intake of SOE staff didn't mean that James Bond came to SIS.
Nonetheless, some of the SOE staff brought in did run clandestine operations within the expanded service.
SOE and SIS
David Smiley, one of the SOE officers who operated behind enemy lines
I met probably the only surviving senior officer who did so - Colonel David Smiley, now 92.
The first time he went into Albania was with SOE in 1943. His task was to organise partisans to fight the Nazis. It was a difficult assignment. The conditions were harsh - and the Albanians reluctant to fight.
In Albania he had poor experience of the "other" service. An SIS agent had "hijacked" one of his supply drops.
"There was no love lost between SOE and SIS" he told me.
At the end of the war, David Smiley was invited to join SIS. As far as he knew, he was one of only three ex-SOE agents to do so.
In the late 1940s he was given another Albanian assignment - his first for SIS.
This time, David Smiley was to train pro-British Albanians and then infiltrate them into the newly Communist state from a base in Malta.
The operation ran from 1949 -1950. His task ended once the Albanians had gone in.
David Smiley was very concerned to discover later that his agents had been quickly captured.
He told me that for years he wondered how it had come about: had one of "his" Albanians been taken and "spilled the beans"?
He later learned that the operation had been betrayed by Kim Philby.
According to Dr Philip Davies, SIS ran similar operations in Ukraine, and the Baltic States.
Pro-British Albanians were trained by the SIS after WWII
The aim was to send in "stay behinds" - nationals of the country, working for the West, who would only become active in case of war. But here too, the networks were betrayed and the agents captured by the communists.
Other files uncovered at the National Archives show that it was the Russian nuclear bomb which put a stop to these adventures.
Once the Russians had tested their device, and there was evidence that they could deploy nuclear weapons, a secret Cabinet committee decided it was too risky to continue with such clandestine operations. Any escalation would be far too dangerous.
This part of history seems to be emerging in fragments - a veteran's recollections here, the odd file coming out at the National Archives.
MI5 - the domestic security service - is making its archive public. So far, the files run to the early 1950s.
By contrast, SIS, or MI6, has no plans to release any part of its own archive. The documents I found were in a folder from the Air Ministry.